1. Field of the Invention
This invention relates generally to multimode engines capable of operating in multiple fueling modes, and, more particularly, relates to a method and apparatus for transitioning between fueling modes in such an engine so as to reduce engine speed fluctuations and/or other undesired responses to such transitions.
2. Discussion of the Related Art
So-called “multimode” engines are capable of operating in multiple fueling modes in that they are powered by different fuels or combinations of fuels depending, e.g., on the prevailing engine speed and load conditions. For example, a dual fuel engine can typically operate in two modes, namely, a “diesel mode” and a “pilot ignited gaseous fuel mode.” In the diesel mode, the engine is fueled solely by a liquid fuel, typically diesel fuel. In the pilot ignited gaseous fuel mode, the engine is fueled primarily by a gaseous fuel, such as natural gas or propane, which is ignited by a relatively small quantity or “pilot” charge of a liquid fuel, typically diesel fuel or engine lube oil.
Depending upon the particular engine utilized, there are typically at least two transition points within the operating range of a dual fuel engine. Specifically, the typical engine is operated in pilot ignited gaseous fuel mode except at the condition that the excess air ratio (lambda) does not permit, such as, (1) very light load under all engine speeds and, (2) at high load, low speed conditions. The transition historically was triggered and controlled based solely as a function of speed and/or load without attempting to achieve a smooth transition. This relatively uncontrolled transition could lead to undesired speed fluctuations. For example, in a prior art dual fuel system, as the vehicle is driving up a hill while operating in pilot ignited gaseous fuel mode, the vehicle's engine speed may lug down sufficiently to trigger a changeover to diesel mode. An uncontrolled rapid switchover to diesel may cause a power surge and a resultant increase in vehicle speed back above the pilot ignited gaseous fuel transition speed for the prevailing load, whereupon the engine switches back to pilot ignited gaseous fuel mode and experiences a power drop. As a result, the vehicle speed may again drop below the transition speed with a resultant switchover to diesel mode. Hence, the engine may switch frequently and repeatedly between operating modes, resulting in noticeable speed surges and droops.
Some prior systems have recognized the problem identified above and have attempted to address it by taking the total energy content of the fuel(s) into account during the transition in an attempt avoid power surges and droops. For instance, U.S. Pat. No. 6,101,986 to Brown (the Brown patent) controls the delivery of diesel and gaseous fuel to the engine during transition between the pilot ignited gaseous fuel mode and the diesel mode to maintain the energy content of combined fuel charge at the desired value of the diesel fuel charge supplied at the end of the transition period. As a result, the quantity of diesel fuel progressively increases during the transition period, while the quantity of gaseous fuel progressively decreases. The process is repeated in a cycle-by-cycle basis until the actual diesel fuel quantity equals the desired quantity for diesel only operation, at which point the transition is considered complete.
U.S. Published Patent Application Serial No. 2002/07816 to Zur Loye similarly discloses a method for controlling a homogenous charge dual fuel engine to switch between diesel only mode and pilot ignited gaseous fuel mode while keeping the total fuel energy content constant.
A problem associated with prior techniques for controlling the transition between operating modes in a multimode engine is that simply maintaining the total fuel energy content constant during the transition period fails to take differences in combustion efficiency into account while air charge parameters remain unchanged. That is, (1) diesel fuel has a lower heating value and a lower stoichiometric air fuel ratio than gaseous fuel per unit fuel mass and, (2) combustion efficiency of pilot ignited gaseous fuel depends on excess air ratio of gas (gas lambda) and ignition timing. Simply increasing or decreasing gaseous fuel quantity may not achieve the desired effect because gas lambda may be outside of an optimal range for the selected gaseous fuel quantity. Existing airflow control devices are incapable of adjusting airflow to the cylinders rapidly enough to immediately obtain the optimum lambda for the selected quantity of the new fuel. As a result, the engine may still exhibit power surges and droops, even if total fuel energy content remains constant.
Another result of the typical dual fuel engine's inability to provide smooth transition between operating modes is that it must set a transition line between diesel mode and pilot ignited gaseous fuel mode at a higher speed than would otherwise be required at a given load, reducing the percentage of total operating range in which the engine is capable of operating in pilot ignited gaseous fuel mode.
The need therefore has arisen to provide a multimode engine that more assuredly provides a smooth transition between operating modes and reduces the frequency of switching between those modes when compared to prior multimode engines. Another need that has arisen, due at least in part to the inability of prior engines to adequately meet the objective for providing a smooth transition between operating modes in a multimode engine, is to adequately maximize the percentage of the engine operating range in which the engine operates in pilot ignited gaseous fuel mode.